It is well known in the video signal processing arts that the signal-to-noise ratio of video signals can be enhanced by recursive filtering. Recursive filtering is the process of integrating a portion of current video signal with a complementary portion of video signals from previous frames or fields of video signals. A typical recursive filter includes a storage element for delaying substantially one field/frame of video signals, a subtracter, a scaler and an added. Current video signal, and delayed video signal from the storage element are applied to respective input ports of the subtracter. The differences output by the subtracter are coupled to the scaler which weights the differences by a coefficient, the value of which is dependent on interscene content. The weighted differences and delayed video signal from the storage element are combined in the adder to produce noise reduced video signal which is applied to the input port of the storage element.
Recursive filtering may be performed on component video signals or on composite video signals. If the process is performed on component video signals, i.e., on luminance signal and chrominance signals separately, using known recursive filters significantly more storage circuitry is required than if the process is performed on composite video signals. On the other hand, if the process is performed on composite video signals, the phase of the chrominance signal of either the current or delayed composite signal must be inverted before the current and delayed signals are combined. In this latter mode a single scaling circuit is employed to weight the composite signal differences resulting in like integration of the luminance and chrominance components of the composite signal.
Michale et al. in U.S. Pat. No. 4,240,106 recognized that it is advantageous to recursively process luminance and chrominance components with different weighting factors because viewer sensitivity to noise differs for the noise in different bands of the video signal frequency spectrum.
It is an object of the present invention to provide a recursive filter which separately processes the luminance and chrominance components of composite video but which requires a storage element no larger than storage elements required for recursively filtering composite video signals.